General Question

How much dish soap do I need to sink a boat?

my grade 5 teacher said that dish soap can change the surface tension of the water and cause water spiders to sink. So I wondered why not a boat? How much dish soap, and what kind of soap do you need to sink a small naval ship?

7 Answers

Spiders are about as heavy as the amount of water they displace. So if they tried to float, they would be mostly under water and unable to breathe. The ones that walk on water do so by carefully placing their feet down so as not to puncture the surface tension of the water, and keeping enough feet in contact with the water surface at all times so that the surface tension is sufficient to support their relatively low body weight. Since soap reduces the surface tension of water (that’s what makes it work in cleaning, it lets the water get to the dirt) soap makes things relying on surface tension unable to use that principle to stay above the surface of the water. They sink to wherever the flotation principle below would establish their waterline. If they are heavier than the amount of water they displace, they go to the bottom.

Boats operate on an entirely different principle from surface tension, the principle of flotation. Even ones made with steel hulls have a great deal of air inside their hulls, so as they sink down in the water, the total weight of water displaced by the hull eventually matches exactly what the boat and its contents weighs. Soap reducing water’s surface tension has zero effect on the flotation principle. If you want to sink a ship with dish soap, fill its entire hull and all the cabin space with the soap and down the ship would go.

As noted, boats don’t depend on surface tension to float. They depend upon the shape of the vessel (and the construction, obviously) to displace water equal to their mass and retain buoyancy because of the pressure-retaining capability of the hull (ability to hold its shape, that is, as the water presses up and the atmosphere, crew and cargo weight press down with equal force).

However, with your speculation about airplanes flying through helium, for example, you’re really onto something.

The way boats float (and airplanes fly) is by pressing down upon the medium that holds them “aloft” (or afloat, in the case of a boat). When airplanes fly into low pressure “pockets” (or downdrafts of air) in the sky… they drop. Fortunately, the drops are usually more or less inconsequential to the aircraft (when they happen at high altitude – if they happen near ground level, planes crash, which is why they won’t land planes during a local thunderstorm), but very disconcerting and sometimes injurious to the passengers. (It’s why the pilot always switches on the Seat Belts sign when the air gets turbulent, because a plane hitting an air pocket or downdraft can drop a hundred feet in a few seconds – which the plane can easily survive – but without a seatbelt you’d find yourself slamming into the ceiling of the plane, which can break bones or worse.)

There is also some speculation that large underwater gas releases can reduce the buoyant quality of seawater. That is, a lot of gas bubbling up through the seabed (we’re talking huge quantities here) make the water so much less dense that ships passing overhead can be sunk into the “fizzy water” and then remain sunk when the fizzing stops and the water returns to its natural density.

Imagine Alka-Seltzer fizzing in a glass of water, and you’ll have some idea of what I mean. As the tablet fizzes and releases carbon dioxide into the water, it is so much less dense that something floating on top of the water in that glass can sink into the fizzing water.

I don’t know if any ship losses have been confirmed in this way, but it’s an interesting speculation.